Video games basically comprise a microprocessor utilized as a central processing unit (CPU), a video display control, a game memory adapted to store a game program and a video memory for storing picture elements of a video display. The picture elements of a display are addressed and fetched from the video memory in accordance with the cathode ray tube (CRT) beam location.
In some cases, the system utilizes a full screen memory, that is, a discrete memory address is provided for every discrete display location of the display, either pixel position or line segment position. Such a system is disclosed in U.S. Pat. No. 4,301,503. In another type of system, a pointer indicative of a display location addresses and reads a picture element from a pattern library. Such a system is disclosed in U.S. Pat. No. 4,243,984. A video display processor (VDP), which together with a video random access memory (RAM) operates in this manner, is marketed by Texas Instruments Corporation as a 9928A video display processor.
A video game display may comprise discrete pixel positions, for example, of 256 positions per line and 192 lines. Thus, any object may be positioned on the screen by reference to a line and column location or a position representative thereof.
To create object movement on the display, in systems using a full screen memory, the addresses of the picture elements are changed periodically, usually during the time of vertical retrace of the display. To create object movement in systems of the second type, the object location in the pointer is changed to a new display location, and the pointer addresses the object in pattern or object libraries at the appropriate beam location.
In either case, the address of the object in the video RAM is changed by the CPU as it receives game player input as to movement of the object. Usually, a game player utilizes a joystick type instrument to control the direction of movement of an object. Such joystick may close switches indicative of coordinate directions of movement. Periodically, the CPU is responsive to the closure condition of the joystick switches. On this particular basis, or some factor thereof, the CPU will rewrite the address of a pattern or object in the video RAM. As the address of the object is periodically changed, the object appears to move on the video display in succeeding displays.
In these systems, movement of the object on the screen is at a rate determined by the game program. The object will move a predetermined number of pixels per display frame. In some games, rate of movement of an object may be variably selected or determined by a game player as disclosed in copending applications Ser. Nos. 430,351, filed Sept. 29, 1982, now U.S. Pat. No. 4,462,594, and Ser. No. 579,709, filed Feb. 13, 1984, now U.S. Pat. No. 4,552,360.
The present invention utilizes the foregoing techniques together with a new method of operating the game to interchangeably store different but related playfield displays in the video memory upon the happening of a given game event. Digital data indicative of at least two playfields are derived from a game memory device in the form of a replaceable game cartridge.